This invention is generally related to a method and system for recovering gas hydrates from subterranean formations. More particularly, this invention relates to a method and system for determining the intrinsic permeability of subterranean formations having gas hydrates sequestered therein.
Permeability of a material is a measure of the material's ability to transmit fluids through its pore spaces and is inversely proportional to the flow resistance offered by the material. Typically, permeability is determined by taking core samples from a hydrocarbon formation and applying permeability measurement techniques to the core samples. When obtainable, cores of the formation provide important data concerning permeability. However, cores are difficult and expensive to obtain, and core analysis is time consuming and provides information about very small sample volumes. In addition, cores, when brought to the surface, may not adequately represent downhole conditions. Thus, in-situ determinations of permeability that can quickly provide permeability information over large portions of the formation would be highly desirable.
Nuclear magnetic resonance (NMR) measurements are used to infer formation permeability. In particular, it is known that the strength of a NMR signal is directly proportional to the number of resonated spins present in a probed volume. Because hydrogen is the nucleus of choice in most borehole measurements, and because NMR tools can be tuned in frequency to resonate a particular nuclear species, the signal amplitude of a tuned tool can be arranged to measure the number of hydrogen atoms in the formation. The number of hydrogen atoms in the formation in turn is related to fluid filled porosity.
In addition to being sensitive to hydrogen density, NMR tools are sensitive to the environment of the hydrogen being probed. For example, hydrogen in a bound or “irreducible” fluid typically has a spin-lattice relaxation time (T2) in the milliseconds to tens of milliseconds, while free or producible fluid has a T2 in the range of tens to hundreds of milliseconds. Thus, in addition to correlating well porosity, the measurements resulting from the NMR sequences applied to a formation provide information which may be correlated with the “free fluid index”, permeability, and residual oil saturation.
Currently, NMR measurements in the borehole are being made via a Combinable Magnetic Resonance tool or “CMR” (a trademark of Schlumberger), and a Magnetic Resonance Expert tool or “MR Scanner” (also a trademark of Schlumberger) which features a gradient magnetic field and multiple frequencies of operation; both of which are commercially successful tools of Schlumberger, the assignee hereof. Details of NMR borehole tools may be seen with reference to U.S. Pat. No. 4,933,638 to Kenyon et al., U.S. Pat. No. 5,023,551 to Kleinberg et al., and U.S. Pat. No. 5,486,761 to Sezginer, all of which are hereby incorporated by reference herein in their entireties.
As disclosed herein, the subject formations may be saturated with hydrates, such as methane hydrates. A gas hydrate is a crystalline solid that is a cage-like lattice of a mechanical intermingling of gas molecules in combination with molecules of water. The name for the parent class of compounds is “clathrates” which comes from the Latin word meaning “to enclose with bars.” The structure is similar to ice but exists at temperatures well above the freezing point of ice. Gas hydrates include carbon dioxide, hydrogen sulfide, and several low carbon number hydrocarbons, including methane. The disclosure herein relates to the recovery of methane from subterranean methane hydrates.
Methane hydrates are known to exist is large quantities in two types of geologic formations: (1) in permafrost regions where cold temperatures exist in shallow sediments and (2) beneath the ocean floor at water depths greater than 500 meters where high pressures prevail. Large deposits of methane hydrates have been located in the United States in Alaska, the west coast from California to Washington, the east coast in water depths of 800 meters, and in the Gulf of Mexico.
A U.S. Geological Survey study estimates that in-place gas resources within gas hydrates consist of about 200,000 trillion cubic feet which dwarfs the previously estimated 1,400 trillion cubic feet of conventional recoverable gas reserves in the United States. Worldwide, estimates of the natural gas potential of gas hydrates approach 400 million trillion cubic feet.
Natural gas is an important energy source in the United States. It is estimated that by 2025 natural gas consumption in the United States will be nearly 31 trillion cubic feet. Given the importance and demand for natural gas the development of new cost-effective sources can be a significant benefit for American consumers.
The determination of permeability and other hydraulic properties of formations surrounding boreholes is very useful in gauging the producibility of formations, and in obtaining an overall understanding of the structure of the formations. For the reservoir engineer, permeability is generally considered a fundamental reservoir property, the determination of which is at least equal in importance with the determination of porosity, fluid saturations, and formation pressure.